Head of security sacked lenient guard (8)
I believe the answer is:
sentinel
'guard' is the definition.
(sentinel is a kind of guard)
'head of security sacked lenient' is the wordplay.
'head of' indicates taking the first letters.
'sacked' is an anagram indicator.
The first letter of 'security' is 's'.
'lenient' is an anagram of 'entinel'.
's'+'entinel'='SENTINEL'
(Other definitions for sentinel that I've seen before include "Watching guard" , "Watcher on guard" , "Look-out, military guard" , "One watching" , "One on guard" .)